1. Field of the Invention
The present invention relates to a ferroelectric memory device serving as a nonvolatile memory for reading data without destructing the data in order to lengthen the service life for the number of read times of a memory cell and further accelerate the access time.
2. Description of the Related Art
In recent years, the significance of a nonvolatile memory in which data can be electrically written or whose data can be erased has been increased in the memory field. Though there are various types of nonvolatile memories, a ferroelectric memory is noticed from the viewpoints of high-speed performance, low-voltage characteristic, and low power consumption. There are various specific configurations of a ferroelectric memory as shown below.
A ferroelectric memory using a ferroelectric capacitor is known which defines two states in accordance with a remanent polarization state in a ferroelectric film. This capacitor detects an internal storage state of 1 or 0 by generating the internal polarization state of 1 or 0 in accordance with the way of applying two types of voltages equal to or higher than the coercive voltage of a ferroelectric thin film and having two different polarities to the ferroelectric capacitor, passing through a storage state due to remanent polarization, and applying voltage equal to or higher than the coercive electric field of the ferroelectric thin film to fetch electric charges. FIGS. 57, 58, 59 are illustrations for explaining the ferroelectric memory.
FIG. 57 is a sectional view showing a structure of a ferroelectric capacitor. As shown in FIG. 57, the ferroelectric capacitor has a structure for holding a ferroelectric thin film 5740 constituted of an inorganic ferroelectric by a first terminal 5741 constituted of a metallic electrode and a second terminal 5742 constituted of a metallic electrode.
FIG. 58 shows a polarization-charge-applied-voltage characteristic of the ferroelectric capacitor shown in FIG. 57. In FIG. 58, curved lines passing through four characteristic points 5801, 5802, 5803, and 5804 show characteristics of applied voltage V and internal polarization charge Q applied between the first terminal 5741 and the second terminal 5742 of the ferroelectric capacitor in FIG. 57.
The characteristic point 5801 shows a state of applying positive high voltage V to the second terminal 5742 from the first terminal 5741 and the characteristic point 5802 shows a state of applying positive high-voltage V to the first terminal 5741 from the second terminal 5742. At the characteristic points 5801 and 5802, internal polarization causes polarization reverse in positive and negative states.
When releasing a potential difference between the first terminal 5741 and the second terminal 5742 of the ferroelectric capacitor under the state of the characteristic point 5801 as 0, the internal polarization is kept as remanent polarization and becomes the state shown at the characteristic point 5804. Moreover, when releasing a potential difference between the first terminal 5741 and the second terminal 5742 of the ferroelectric capacitor under the state of the characteristic point 5802 as 0, internal polarization is kept as remanent polarization and becomes the state shown at the characteristic point 5803.
Thus, because the ferroelectric capacitor has the hysteresis characteristic shown in FIG. 3, it releases terminals at the both ends of a ferroelectric capacitor and has different remanent polarization depending on a previous state even if setting voltage to 0. The remanent polarization can store data by corresponding to the characteristic points 5803 and 5804.
When applying voltage V (ΔVB) to the second terminal 5742 on the basis of the first terminal 5741 from a state in which the terminals at the both ends of the ferroelectric capacitor are released, the characteristic point moves to the characteristic point 5801. In this case, when the previous state is at the characteristic point 5803, electric charges of ΔQHB shown in FIG. 58 are fetched, while when it is at the characteristic point 5804, electric charges of ΔQLB are fetched. As shown in FIG. 58, because ΔQLB<<ΔQHB, it is possible to determine the previous state stored as remanent polarization as 1 or 0 in accordance with the fetched number of electric charges.
The circuit shown in FIG. 59 is known as a specific circuit for performing the above operations.
As shown in FIG. 59, this circuit is constituted of a ferroelectric capacitor 5911 and an N-type insulating-gate field-effect transistor (hereafter referred to as MOSFET) 5912 and includes a word line (WL) 5913, bit line (BL) 5914, and plate line (PL) 5915. In this case, the MOSFET is an abbreviation of a Metal-Oxide-Semiconductor Field-Effect Transistor.
More minutely, the word line 5913 is connected to the gate electrode of the MOSFET 5912. The bit line 5914 is connected to an electrode serving as a source or drain of the MOSFET 5912. Moreover, the plate line 5915 is connected to one end of the ferroelectric capacitor 5911. The other end of the ferroelectric capacitor 5911 is connected to an electrode serving as a drain or source of the MOSFET 5912.
The circuit in FIG. 59 constituted as described above supplies potential to be applied to the ferroelectric capacitor 5911 to the bit line 5914 and plate line 5915 and turns on/off the MOSFET 5912 by the word line 5913 and thereby, performs the write operation and read operation of the above described electric charges.
In this case, the above method fetches electric charges when reading data. That is, because data is destructed, the above method is a method generally referred to as destructive read and an example of this method is disclosed in JP11-39882A (hereinafter referred to as Patent Document 1).
There is a method referred to as a nondestructive read in which data is not destructed when it is read. By improving a material of a ferroelectric thin film, there is a method for detecting a difference between output electric charges due to a difference between operating points when a weak voltage is applied to a ferroelectric capacitor due to a difference between inclinations of characteristics corresponding the characteristic points 6003 and 6004 shown in FIG. 60 by assuming the polarization-charge-applied-voltage hysteresis characteristic to be asymmetric as shown in FIG. 60. Examples of this method are disclosed in JP2-198094A and JP5-82800A (hereinafter referred to as Patent Documents 2 and 3).
Moreover, as shown in FIGS. 61 and 62, there is a structure in which a ferroelectric thin film 6100 is formed at the gate portion of a field-effect transistor, voltage equal to or higher than the coercive voltage of the ferroelectric thin film 6100 is applied between a gate electrode 6101 and a substrate 6109 or to a source electrode 6102 and drain electrode 6103 to make the ferroelectric thin film 6100 generate polarization so as to store data in accordance with the state of remanent polarization even after the applied voltage is removed. This can detect a written polarization direction, that is, a difference of 1 or 0 because electric charges induced by the channel of a field-effect transistor depend on the remanent polarization and become a difference between threshold voltages, and flowing current values are different.
A field-effect transistor having a ferroelectric thin film at its gate portion may be referred to as MFSFET. In this case, the MFSFET is an abbreviation of a Metal-Ferroelectrics-Semiconductor Field-Effect Transistor.
In FIG. 61, potential 0 is applied to the gate electrode 6101 through the word line 6104 and a positive potential V equal to or higher than coercive voltage is applied to the source electrode 6102 and drain electrode 6103 through the first bit line 6105 and second bit line 6106, and the ferroelectric thin film 6100 causes the polarization of a positive electrode at the gate side and the polarization of a negative electrode at the substrate side.
Moreover, in FIG. 62, positive potential V equal to or higher than coercive voltage is applied to the gate electrode 6101 through the word line 6104 and potential 0 is applied to the source electrode 6102 and drain electrode 6103 through the first bit line 6105 and second bit line 6106, and the ferroelectric thin film 6100 causes negative-electrode polarization at the gate side and positive-electrode polarization at the substrate 6109 side.
When reading data, there is a method for detecting a difference between currents flowing through an MFSFET by using a difference between the remanent polarizations as a change in threshold voltages of the MFSFET. As an example of this, there is JP2002-543627A (hereinafter referred to as Patent Document 4).